Caption: Immanuel Kant (1724-1804), German philosopher. Kant published various influential philosophical works on the natural sciences, particularly geophysics and astronomy. In 1755, during work on Newtonian cosmology, Kant introduced the nebular theory of solar system origins. He predicted the existence of the planet Uranus before its discovery by Herschel. Another influential theory was his belief in two fundamental forces: gravitational (attractive) and elastic (repulsive). The latter needed a medium through which to act whilst the former did not. Kant also rejected the theory that matter was indivisible. This wood engraving dates from circa 1888.